The present invention relates to an optical-axis directional indicating apparatus to be used in adjustments to the optical axis of an optical wireless communications system.
In optical wireless communications systems, a photoemitter transmits an infrared beam modulated with data signals and then a photoreceptor receives and demodulates the modulated infrared beam, thus achieving data transmission.
Higher transfer speed requires larger amount of light received at the photoreceptor, which further requires a focused beam from the photoemitter for higher light intensity. These requirements are satisfied by optical-axis adjustments between the photoemitter and the photoreceptor.
A known optical wireless communications system, such as illustrated in FIG. 1, employs optical-axis adjustments using a pilot beam.
In FIG. 1, the optical wireless communications system consists of a base unit 1 and a local unit 2.
The base unit 1 has a photoemitter 3 for emitting a data beam (carrying data) for data transmission, a pilot-beam emitter 4 for emitting a divergent beam (pilot beam) having a frequency different from that of the data beam, and a data receptor 5 for receiving data beam transmitted from the local unit 2.
The local unit 2 has a photoemitter 6 and a photoreceptor 7 both rotatable in a horizontal and also a vertical direction. It receives the pilot beam emitted by the pilot-beam emitter 4 of the base unit 1 while the photoemitter 6 and the photoreceptor 7 are rotating in the horizontal and vertical directions. The photoemitter 6 and the photoreceptor 7 stop at the maximum level of received light for optical-axis adjustments between the base unit 1 and the local unit 2. The photoreceptor 7 receives both of the data beam and the pilot beam, which may, however, be separated into two receiving sections to receive the data beam and the pilot beam separately.
As illustrated in FIGS. 2A to 5B, the photoreceptor 7 of the local unit 2 has a quadrant PD (photodiode) 8 having 2×2 PD cells in a horizontal and a vertical direction. Four PD cells PD_A PD_B, PD_C and PD_D are arranged into a 2×2 matrix in a package. Each PD cell performs photoelectric conversion to detect an electric signal carried by the data or pilot beam.
FIG. 6 shows a circuit block diagram installed in the local unit 2 of the known optical wireless communications system (FIG. 1), for rotating the unit 2 in the horizontal and vertical directions in optical-axis adjustments.
The operation of the circuit diagram is explained with reference to a flowchart shown in FIG. 7.
Step S1 at the photoreceptor 7: A pilot beam transmitted from the base unit 1 is received at the photoreceptor 7 of the local unit 2. Electric signals (Dir_A Dir_B, Dir_C and Dir_D) are detected from the pilot beam by the PD cells PD_A PD_B, PD_C and PD_D of the quadrant PD 8. The electric signals are converted into voltages and amplified by amplifiers 9.
Steps S2 to S6 at a controller 10: The amplified signals (Dir_A Dir_B, Dir_C and Dir_D) are selected by a switch 11 under control by a microcomputer 14. Each selected signal is amplified by an amplifier 12 and detected by a level detector 13 for its amplified DC level. The detected DC levels are sequentially supplied to the microcomputer 14 and compared with each other.
Steps S7 to S12 at a motor unit 15: A tilt (vertical) motor Mt and a pan (horizontal) motor Mp are driven by the microcomputer 14 to rotate the quadrant PD 8 so that the PD cells PD_A PD_B, PD_C and PD_D can receive the same amount of light.
Steps S5 to S12 are explained in detail.
Optical-adjustments in the vertical direction TILT:
(1) Comparison is made between addition of the levels of light received at the PD cells PD_A and PD_D and addition of the levels of light received at the PD cells PD_B and PD_C (step S5).
(2) If the former addition and the latter addition are equal to each other in step S5, the spot of pilot beam must have been received at the center of the quadrant PD 8 in the vertical direction TILT (step S5→step S9).
(3) If the former addition is larger than the latter addition in step S5, the spot of pilot beam must have been received at the upper section of the quadrant PD 8, as illustrated in FIGS. 2A and 2B. The tilt motor Mt is driven to tilt the quadrant PD 8 upwards, as indicated by an arrow in FIG. 2B. (step S6→step S7).
(4) If the former addition is smaller than the latter addition in step S5, the spot of pilot beam must have been received at th lower section of the quadrant PD 8, as illustrated in FIGS. 3A and 3B. The tilt motor Mt is driven to tilt the quadrant PD 8 downwards, as indicated by an arrow in FIG. 3B. (step S6→step S8).
(5) The procedures (3) and (4) are repeated so that the spot of pilot beam can be received at the center of the quadrant PD 8 in the vertical direction TILT.
Optical-adjustments in the horizontal direction PAN:
(6) Comparison is made between addition of the levels of light received at the PD cells PD_A and PD_B and addition of the levels of light received at the PD cells PD_C and PD_D (step S9).
(7) If the former addition and the latter addition are equal to each other in step S9, the spot of pilot beam must have been received at the center of the quadrant PD 8 in the horizontal direction PAN (step S9→END).
(8) If the former addition is larger than the latter addition in step S9, the spot of pilot beam must have been received at the right section of the quadrant PD 8, as illustrated in FIGS. 4A and 4B. The pan motor Mp is driven to turn the quadrant PD 8 right, as indicated by an arrow in FIG. 4B. (step S10→step S11).
(9) If the former addition is smaller than the latter addition in step S9, the spot of pilot beam must have been received at the left section of the quadrant PD 8, as illustrated in FIGS. 5A and 5B. The pan motor Mp is driven to turn the quadrant PD 8 left, as indicated by an arrow in FIG. 5B. (step S10→step S12).
(10) The procedures (8) and (9) are repeated so that the spot of pilot beam can be received at the center of the quadrant PD 8 in the horizontal direction PAN.
The known optical wireless communications system employs the automatic optical-axis adjusting technique explained above so that a user can easily set the local unit 2. Thus, this system requires a microcomputer and the peripheral circuitry for automatically driving gears of the motors Mt and Mp, etc., which makes the local unit 2 bulk and expensive.
Another known optical wireless communications system employing a manual optical-axis adjustment technique is disclosed in Japanese Unexamined Patent publication No. 7 (1995)-131422. In this optical-axis adjustment technique, the level of transmitted light is detected and displayed on a level monitor.
Monitoring the level of transmitted light on display or by sound is applied to adjustments to the direction of an antenna of a TV set towards an antenna of a TV station.
Monitoring the level of transmitted light on display or by sound like disclosed in the above Japanese Unexamined Patent publication, however, forces a user to adjust the optical axis upwards, downwards, right or left by cut and try because he or she dose not know in which direction the optical axis should be adjusted, which is troublesome and takes much time.